With the market expansion of mobile electronic devices such as cellular phones, laptop personal computers
and digital cameras, a long-life secondary battery having high energy density to be able to have higher output is desired as a cordless power source of these electronic devices.
To respond to such requirements, secondary batteries, which use alkali metal ions such as lithium ions as a charge carrier, and use an electrochemical reaction associated with giving and receiving of charges of the charge carrier, have been developed. Particularly, a lithium ion secondary battery has high energy density, and become widely available as a battery for automobile use.
An electrode active material of structural elements of the secondary battery is a substance directly contributing to electrode reactions of charging and discharging in the battery, and has a central role in the secondary battery. That is, the electrode reaction in the battery is a reaction which occurs associated with giving and receiving of electrons by applying a voltage to an electrode active material electrically connected to an electrode located in an electrolyte, and the electrode reaction progresses during charge and discharge of the battery. Accordingly, as described above, the electrode active material systemically has a central role in the secondary battery.
In the lithium ion secondary battery, a lithium-containing transition metal oxide is used as a positive electrode active material and a carbon material is used as a negative electrode active material, and charge and discharge is performed by using a lithium ion insertion and detachment reactions for these electrode active materials.
However, the lithium ion secondary battery has a problem that a charge-discharge rate is restricted since the movement of lithium ions in the positive electrode becomes rate-determining. That is, since the moving rate of lithium ions in the transition metal oxide of the positive electrode in the lithium ion secondary battery is slow as compared with the electrolyte or the negative electrode, the electrode reaction rate in the positive electrode becomes rate-determining to restrict a charge-discharge rate, and consequently there are limitations to an increase in output or shorten a charging time.
Thus, in recent years, secondary batteries using an organic radical compound, an organic sulfur compound or a quinone compound for an electrode active material are actively researched and developed in order to solve these problems.
For example, the document 1 is known as a prior art document in which the organic radical compound is used for an electrode active material.
The document 1 proposes an active material for a secondary battery which uses a nitroxyl radical compound, an oxyradical compound, and a nitrogen radical compound having a radical on a nitrogen atom.
The organic radical compound can increase the reaction site concentration because unpaired electrons to be reacted exist locally in radical atoms, and thereby, realization of a secondary battery having high capacity can be expected. Further, since radicals have a large reaction rate, it is considered that charging can be completed in a short time by charge and discharge with the use of an oxidation-reduction reaction of stable radicals.
Further, the document 1 describes Examples in which a nitroxyl radical having high stability as a radical is used, and for example, it is verified that when an electrode layer containing a nitronylnitroxide compound is used as a positive electrode, and a copper foil having lithium attached thereto is used as a negative electrode to prepare a secondary battery, and charge and discharge are repeated, charging and discharging can be performed over 10 cycles or more.
Further, the documents 2 and 3 are known as a prior art document in which the organic sulfur compound is used for an electrode active material.
The document 2 proposes a novel metal-sulfur type battery in which the organic sulfur compound serving as a positive electrode material has an S—S bond in a charged state, the S—S bond is cleaved during discharge of the positive electrode to form an organic sulfur metal salt having metal ions.
In the document 2, as the organic sulfur compound, a disulfide-based organic compound (hereinafter, referred to as a “disulfide compound”) indicated by the general formula (1′) is used.R—S—S—R  (1′)
R indicates aliphatic organic groups or aromatic organic groups, and the aliphatic organic groups or the aromatic organic groups may be the same or different from each other.
In the disulfide compound, a two-electron reaction can occur, and an S—S bond of the compound is cleaved in a reduced state (discharged state), and thereby organic thiolate (R—S—) is formed. The organic thiolate forms an S—S bond in an oxidized state (charged state), and returns back to the disulfide compound indicated by the general formula (1′). That is, since the disulfide compound forms the S—S bond having small bond energy, a reversible oxidation-reduction reaction occurs with the use of bonding and cleavage by the reaction, and thereby charge and discharge can be performed.
Further, the document 3 proposes an electrode for a battery which has a structural unit indicated by the following formula (2′):—(NH—CS—CS—NH)  (2′)
and includes a rubeanic acid or rubeanic acid polymer capable of being bonded with lithium ions.
The rubeanic acid or rubeanic acid polymer containing a dithione structure indicated by the general formula (2′) is bonded with lithium ions during reduction, and releases the bonded lithium ions during oxidation. It is possible to perform charge and discharge by using such a reversible oxidation-reduction reaction of rubeanic acid or rubeanic acid polymer.
In the document 3, when the rubeanic acid is used for the positive electrode active material, a two-electron reaction can occur, and a secondary battery having a capacity density of 400 Ah/kg at normal temperature is obtained.
Further, the document 4 is known as a prior art document in which the quinone compound is used for an electrode active material.
The document 4 proposes an electrode active material containing a specific phenanthrenequinone compound having two quinone groups at relative ortho positions.
The specific phenanthrenequinone described in the document 4 initiates a two-electron reaction peculiar to a quinone compound with moving carriers and can generate a reversible oxidation-reduction reaction. Moreover, the oligomerization or polymerization of the specific phenanthrenequinone compound achieves insolubility in an organic solvent without causing a decrease in the number of reaction electrons due to the repulsiveness between electrons. Further, the document 4 indicates that a phenanthrenequinone dimer exhibits two oxidation-reduction voltages (around 2.9 V and around 2.5 V), and that the first discharge capacity reaches 200 Ah/kg.
The document 1: JP No. 2004-207249 A (pars. [0278]-[0282])
The document 2: U.S. Pat. No. 4,833,048 (claim 1, fifth column lines 20-28)
The document 3: JP No. 2008-147015 A (claim 1, par. [0027], FIGS. 3 and 5)
The document 4: JP No. 2008-222559 A (claim 4, pars. [0027], [0033], FIGS. 1 and 3)